1. Technical Field of the Invention
The present invention relates generally to a spark plug which produces a series of sparks to ignite an air-fuel mixture and a production method thereof, and more particularly to such a spark plug designed to ensure a high strength of joint between a ground electrode and a metal shell and a production method thereof.
2. Background Art
US 2002/0063504 A1 (Japanese Patent First Publication No. 2002-222686) discloses a spark plug which has an Ir (iridium) alloy-made ground electrode joined directly to an end surface of a metal shell by laser welding. U.S. Pat. No. 6,307,307 B1 and U.S. Pat. No. 6,373,172 B1 (Japanese Patent First Publication No. 2001-210447) propose spark plugs which include an Ni base alloy-made support joined to a metal shell and an Ir alloy-made ground electrode joined to the support by laser welding. The ground electrode contains 50 Wt % or more of Ir and is embedded in the support.
When the ground electrode is joined to the metal shell using typical laser welding techniques, a weld area between them is usually small, which could result in a lack of the strength of the joint between the ground electrode and the metal shell. Similarly, the weld between the metal shell and the Ni base alloy-made support may lack the mechanical strength because of restriction on a weld area. Usually, the laser welding results in a considerable rise in temperature of a weld during exposure to laser beams. After the laser exposure, the weld is cooled rapidly, thus increasing the possibility of solidification cracking thereof, which results in a lack of the strength of the joint between the ground electrode and the metal shell.
In recent years, modern automotive vehicles have been required to meet high power, low fuel consumption, and low exhaust emissions requirements, thus resulting in an increase in temperature of burning atmosphere in the engine. Therefore, when the weld between the metal shell and the ground electrode has already experienced the solidification cracking, and the ground electrode which is usually subjected to the most intense heat in the combustion chamber of the engine rises in temperature thereof greatly, it may cause the weld to be cracked completely, thus resulting in separation of the ground electrode from the metal shell.